1. Field
The present disclosure is directed to a method and apparatus for channel determination for time division duplex systems with coupled antennas.
2. Introduction
Presently, wireless communication devices communicate with other communication devices using wireless signals. Many wireless communication devices have multiple antennas that can transmit more focused signals to a receiving device using antenna beamforming. It is generally assumed that Time Division Duplex (TDD) systems have an advantage over Frequency Division Duplex (FDD) systems in that adaptive transmit beamforming can be implemented without feedback from the receiver by exploiting channel reciprocity. Generally speaking, if channel reciprocity holds, then the channel observed at the j-th User Equipment (UE) antenna when transmitting from the i-th base station, such as an enhanced NodeB (eNB), antenna is the same as the channel observed at the i-th eNB antenna when transmitting from the j-th UE antenna. Thus, if reciprocity holds, the matrix channel observed at the UE when transmitting from the eNB, HeNB→UE, and the matrix channel observed at the eNB when transmitting from the UE, HUE→eNB, have the relationHUE→eNB=HeNB→UET.If the eNB has M antenna and the UE has N antennas, then the matrix HeNB→UE has dimension M×N, and HUE→eNB has dimension N×M.
An issue that has not been considered is that when there is coupling within the antenna array, the channel between the eNB and the UE can be a function of both the circuits used to drive the transmit array and the impedances used to load the receive array. Thus, there is a need for a method and apparatus for channel determination for time division duplex systems with coupled antennas.